Electrical cabinets are often used for housing various types of electrical and electronic equipment at outdoor and indoor locations. These cabinets have several functions, which typically include the avoidance of electrical safety hazards by preventing unauthorized contact with the equipment, and the protection of the equipment itself from tampering or accidental damage. In the case of electrical cabinets installed at outdoor locations, an additional function of the cabinet may be to protect the equipment from damage caused by precipitation, moisture, solar heating, and other environmental conditions.
In one common application, outdoor electrical cabinets are employed by local telephone companies to house the channel banks which carry out analog-to-digital and digital-to-analog conversion between telephone company lines and individual subscriber lines. In situations where fiber optic telephone lines are used, the cabinets may also house interface circuits for coupling the fiber optic lines with conventional metallic conductors leading to the subscribers' telephone equipment. In all of these applications, there is a desire to make the cabinets as small and as low in height as possible for aesthetic reasons, since they are often installed in residential neighborhoods. The increasing miniaturization of electronic components has made smaller cabinets possible.
Most types of electronic equipment generate significant amounts of heat which must be dissipated in order to prevent the equipment from overheating and possibly malfunctioning. When the equipment is housed in a cabinet, the most straightforward approach for providing cooling is to maintain a flow of air between the interior and exterior of the cabinet through vents or louvers. This may be accomplished through natural convection, but fans may be required when the equipment is tightly packed within the cabinet and does not allow sufficient air flow for convective cooling. Unfortunately, the introduction of outside air is undesirable in many instances, particularly when the air contains contaminants that are detrimental to the operation of the electrical equipment, such as salt spray or agricultural chemicals. Although various types of filters can be employed to reduce the infiltration of contaminants, these filters are not completely effective and require periodic cleaning or replacement.
In order to maintain the cleanest possible environment within an electrical cabinet, the cabinet can be designed in such a manner that it is closed or sealed with respect to the outside air. This effectively eliminates problems caused by airborne contaminants and other environmental conditions, but it complicates the problem of cooling since it is no longer possible to maintain an air flow between the interior and exterior of the cabinet in order to dissipate heat. If the cabinet is sufficiently large and the equipment inside the cabinet is not too densely packed, sufficient cooling may be provided by natural convection currents within the cabinet and by heat transfer through the walls of the cabinet. However, given the current trend toward smaller cabinets and more tightly packed equipment, these conditions are usually not present.
Several attempts have been made to devise air-to-air heat exchangers for use in electrical cabinets, with the goal of maintaining a closed or sealed environment within the cabinet while providing adequate cooling for the electrical components enclosed in the cabinet. In U.S. Pat. No. 5,040,095, to Beaty et al, cooling of a sealed telephone equipment cabinet is provided by means of a jacket which encompasses the top and two sides of a sealed compartment within the cabinet and forms a cooling air passage around the compartment. The roof of the compartment is corrugated for increased heat dissipation, and cooling air is forced between the corrugated roof and the outer jacket to cool the interior compartment. Although this arrangement has the advantage of allowing the interior compartment to remain closed with respect to the outside air, the irregularity of the corrugated roof surface makes it difficult to form an effective seal between the roof and the walls of the compartment. Moreover, because the corrugated surface inherently presents the same surface area on the top and bottom, the amount of heat transfer is equivalent to that obtained with a flat sheet (albeit one of somewhat larger dimensions) and cannot be optimized based on the amount of air flow that would be expected to occur on each side. Finally, since the cooling air enters and exits the outer jacket through vents or louvers located near the base of the cabinet, a purely convective air flow is not practical and a fan is required to maintain a sufficient flow of air.
Another approach to the problem of maintaining a closed or sealed environment within an electrical cabinet is exemplified by U.S. Pat. No. 4,949,218, to Blanchard et al. In this patent, a pair of cooling fin assemblies are mounted in the upper interior portion of the cabinet in order to serve as a heat exchanger. Each cooling fin assembly consists of a plurality of horizontally elongated fins which are spaced apart by means of spacer bars, with each fin extending both above and below the spacer bars. The spacer bars extend the full horizontal length of the fins, and create a solid wall which divides the upper portion of each fin from the lower portion. The cooling fin assemblies are installed in an air duct which extends horizontally across the top of the cabinet, with the spacer bars forming the lower wall of the air duct and the lower portions of the fins extending into the sealed cabinet area below. Fans are mounted at opposite ends of the air duct to cause outside air to flow horizontally through the upper portions of the fins, and the lower portions of the fins absorb heat from the interior of the cabinet. Although this arrangement provides isolation between the interior and exterior of the cabinet, the cooling fin assemblies are relatively complex and difficult to fabricate, adding significantly to the cost of the cabinet. Also, the horizontal orientation of the air duct makes it impractical to rely on a purely convective flow of outside air, and hence fans are required for efficient cooling. A further disadvantage is that the air duct, cooling fin assemblies and fans add significantly to the height of the cabinet, making it difficult to achieve a low cabinet height that is often desired for aesthetic reasons.
In summary, the prior art has failed to provide a simple, inexpensive and effective heat exchanger construction which allows a closed environment to be maintained within an electrical cabinet. The arrangements proposed so far, as exemplified by those discussed above, are relatively complex in construction and are generally not readily adaptable to conventional types of cabinet designs without substantial modification. Also, the existing arrangements are not practical for use with purely convective air flows, and hence additional complexity is introduced because of the need for powered fans to maintain the required air flow.